This technology relates generally to flexible actuators. In particular, this invention relates to soft robotic manipulators.
An example of a “robot” is an automatically controlled, programmable, multipurpose manipulator. A robot can perform its function at a fixed location, or in motion. Robotics is a field of enormous (and growing) importance, in fields from assembly to surgery. Most robotic systems are “hard”, that is, composed of metallic structures with joints based on conventional bearings. These structures are often modeled after animal limbs (although structures not found in nature—for example, wheels and treads—are also common in mobile robots).
In an effort to build robots that can carry out sophisticated tasks in unstructured environments, researchers continue to emulate living creatures and their materials, morphology and movements. Over the last several years, soft robotic manipulators have generated significant interest due to their wide range of potential applications that are challenging for “hard” robots. For example, soft robots can handle delicate objects such as eggs because the surface of soft robots can conform to the shape of the handled objects. Soft robots can also fit into places that are challenging for hard robots. For instance, a soft robot can fit under a door jam by deflating itself. In addition, soft robots can move in an environment that are challenging for hard robots. For instance, soft robots can maneuver on non-stiff surfaces, such as mud, clay, jello, or in fluids such as water.
One way to build a soft robotic manipulator is by integrating rigid robotic skeletons with soft skins or compartments. However, these structures can only move in limited ways. In addition, rigid skeletons may not be suited for many applications, such as manipulating delicate objects or objects with significant part-to-part variance.
Pneumatic artificial muscles, such as McKibben actuators, are contractile or extensional devices operated by pressurized air. McKibben-type actuators have a simple structure consisting of an internal bladder wrapped in a braided mesh shell. The braided mesh shell includes flexible yet non-extensible threads oriented at a bias around the bladder. When the internal bladder is pressurized, the pressurized air pushes against the inner bladder surface and external shell, causing the bladder to expand. Like the Chinese finger puzzle, the braided mesh shell shortens in a scissor-like action due to the non-extensibility of the threads. As the braided mesh shell shortens, the actuator shortens accordingly, thereby exerting a force in the direction of contraction. These actuators can be fast and can have length-load dependence similar to that of muscles, but possess only one mode of actuation—contraction and extension.
Soft robots, or soft robotic actuators, can be most easily identified by the materials used in their manufacture and their methods of actuation. The field of soft robotic actuation began with work by Kuhn et al in 1950. Kuhn et al focused on the reversible change of a polymeric material, namely the coiling and uncoiling. The reversible change of a polymeric material depends on the acidity of the surrounding medium. Kuhn et al leveraged this property to successfully move a weight. This demonstrated the possibility of using soft materials in robotic actuation. Hamlen et al extended this idea in 1965 and showed that polymeric materials can be contracted electrolytically.
Kuhn et al and Hamlen et al set the scene for using the polymeric gels for soft robotics. In particular, Otake et al demonstrated the use of electro-active polymers in the manufacture of starfish-shaped robotic actuators. Also, in 1996, Suzumori et al demonstrated pneumatically-driven soft actuators. These actuators were configured to respond to pressurization of sealed chambers fabricated from extensible polymers. This type of actuation has been used on the millimeter scale to fabricate grippers, tentacles, and other related devices including pneumatic balloon actuators.
There is a need for larger portions of robots to be composed of soft materials. There is also a need for actuators that provide more and greater modes of movement.